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Abstract Summary The loss of coronal tooth structure for pathologic reasons can be related to a multitude of individual and/or combined etiologic factors related to oral and dietary habits which ultimately cause abrasion and/ or erosion of enamel and dentin. With the pattern and extent of damage to severely worn teeth, the restoration of these defects becomes increasingly demanding and often requires multiple alterations to preparation design. Therefore, a conservative approach as part of “minimally invasive dentistry” in both the preparation and restoration design is emerging. To achieve such a conservative preparation; a restorative material will be needed to attain enough strength while allowing minimum thickness. These two properties can be attained with the use of modern ceramics and computerized restorative dentistry (CAD/CAM). Henceforth, the present study was aimed at studying posterior occlusal veneers as a recent treatment option for restoring worn occlusal surfaces. The use of various preparation designs and modern ceramics milled with CAD/CAM machinery was investigated. Marginal adaptation and fracture resistance were investigated in an attempt to give an idea about the durability of the restoration under laboratory conditions. The aim of this study was to evaluate marginal adaptation and Fracture resistance of occlusal veneers with two different preparations: the standard occlusal preparation and standard occlusal preparation with a Summary 109 buccal veneer extension ‘vonlay’; using different ceramic materials: Lithium disilicate ceramic, hybrid ceramic and Translucent solid zirconia. Sixty extracted, archived, sound and non-carious human mandibular molars were selected using the average crown dimensions. The selected teeth were divided into two main groups according to the preparation of the occlusal veneer where group (O) received a standard occlusal reduction in enamel following the anatomical form with an angle of 150o between the cusps and group (OV) that received a standard occlusal reduction in enamel following the anatomical form with an angle of 150o between the cusps with a buccal extension ending with a chamfer finish line “vonlay”. Each group of teeth was then subdivided into 3 subgroups according to the material of the occlusal veneer they received where Subgroup (a) teeth received lithium disilicate ceramic, Subgroup (b) received hybrid ceramic and Subgroup (c): teeth received translucent solid zirconia. All molars were restored with standardized anatomical occlusal veneers, of 1.5 mm minimum thickness at the position of the central fossa and the two occlusal planes, buccal and lingual, with a 150o angle between them. These restorations were generated with the help of CAD/CAM system. Self-adhesive resin cement was applied on the occlusal surface of the occlusal veneers after each restoration received its proper and strict Summary 110 surface treatment according to the manufacturer’s instructions. The fabricated occlusal veneers were then cemented onto their corresponding teeth. All specimens were stored in distilled water in an incubator at 37 o C for 3 days. Samples were evaluated for marginal adaptation. After initial marginal adaptation evaluation, the samples were subjected to thermocycling in a deionized water bath with 10,000 cycles between 5°C and 55°C, with a dwell time of 15 seconds and a 15 seconds transfer time between baths to simulate temperature fluctuations in the oral cavity. The cemented samples were evaluated again for marginal adaptation after thermocycling using microcomputed tomography. These samples were then individually mounted on a computercontrolled material testing machine with a loadcell of 5kN and data were recorded using computer software. The load for fracture was recorded in Newtons. All the data collected from marginal adaptation and fracture resistance results was tabulated and statistical analysis was performed. For marginal adaptation, all the samples showed a volumetric gap increase after thermocycling, but all the values were within the clinically recorded reference values. The vonlay preparation recorded statistically significant higher marginal gap values than the standard occlusal preparation. Regarding the material, monolithic solid zirconia gave the Summary 111 highest marginal gap values, lithium disilicate followed after and the least values were recorded with hybrid ceramic. For fracture resistance, regardless of the material of the occlusal veneer, there was no statistical significance between the two different preparations. As for the material; again; statistically significant results were noted between monolithic solid zirconia and the other two materials, lithium disilicate followed by hybrid ceramic. However, no statistical significance was found between lithium disilicate and hybrid ceramic restorations. 112 Conclusion Within the limitations of the study, the following conclusions could be drawn: • Thermocycling resulted in an increase in the marginal gap value regardless of the preparation design or the ceramic material. • All samples showed marginal gap values that were clinically acceptable within the range of 150 µm whether before or after thermocycling. • Increasing the area of the preparation results in an increase in marginal gap, as well as the fracture resistance of the restoration. Clinical recommendations: - Hybrid ceramics can be considered a promising alternative to lithium disilicate for they offer better marginal adaptation values. In case of high occlusal forces, Bruxzir should be considered. - Both the vonlay preparation and the standard occlusal veneer preparation can be considered as treatment options in supporting the tooth structure against fracture when high forces are expected. Conclusions 113 Suggestions for further investigations: 1. The use of a chewing simulator should be considered since it gives a better and more accurate representation of the parafunctional |